This invention relates generally to the measurement of feature dimensions, and specifically, it relates to the measurement of feature dimensions without manual intervention using dark-field illumination to distinguish the edges of a feature and using image processing techniques to measure the disparity between opposing feature edges.
Dark-field microscopy is a technique in which an illumination source provides a convergent cone of light at a low angle to the plane of the specimen, centered on and symmetrical about the optical axis of the imaging system. The image is formed by collecting and focusing only that light which is scattered from topographic feature edges on the specimen. Since flat areas reflect rather than scatter, they appear dark in the image, while feature edges appear light.
In the prior art, techniques are known for extracting the dimensional information from the images generated by dark-field microscopy without manual intervention. Generally, these known techniques analyze the image to discriminate the opposing feature edges and calculate the distance between the opposing edges. These techniques use either a single scan line from the image or the average of multiple scan lines to reduce the effects of high-frequency noise. Typically, a target in the shape of a single rectangle is scanned and the resulting data is analyzed to identify the opposing edges. By focusing attention on detailed analysis of the feature edges, however, such prior art techniques have certain inherent limitations. Firstly, since the information representing the details of the edges are carried in the high spatial frequency region of the image spectrum, limited and laborious techniques for high frequency noise reduction are required. Secondly, the results are sensitive to the details of the contrast formation mechanism of specific samples and tend to vary from one type of specimen to another or even for different specimens of the same type. Thirdly, the contrast formation is sensitive to the illumination and other parameters of the imaging equipment, such as focus, so that results from a single specimen will be different depending on the settings.